1. Field of the Invention
This invention relates to a method for catalytic hydrogenation effected on a polymer of a conjugated diene for the purpose of imparting to the polymer an ability to resist weather conditions, oxidation, heat, etc. More particularly, this invention relates to a novel method for preferentially hydrogenating unsaturated double bonds in conjugated diene units of the polymer under mild hydrogenation conditions by the use of a polymer hydrogenation catalyst comprising at least one specific titanium compound.
2. Description of the Prior Art
Generally, polymers obtained by polymerizing or copolymerizing conjugated dienes are widely utilized for commercial purposes. These polymers have unsaturated double bonds as residues in their polymer chains. These unsaturated double bonds are advantageously utilized as for vulcanization and yet have a disadvantage that they are deficient in stability to resist weather conditions, oxidation, etc. Particularly, block copolymers obtained from conjugated dienes and vinyl-substituted aromatic hydrocarbons are used in their unvulcanized state as thermoplastic elastomers and transparent impact-resistant resins or as modifiers for styrenic resins and olefinic resins. Owing to the unsaturated double bonds present in their polymer chains, however, such block copolymers are deficient in resistance to weather conditions, oxidation, and ozone. In the field of exterior materials to which such properties are indispensable, therefore, the block copolymers find limited utility because of this drawback.
This deficiency in stability can be notably improved by hydrogenating such a block copolymer and consequently eliminating the unsaturated double bonds persisting in the polymer chain thereof. Numerous methods have been so far proposed for hydrogenating hydrocarbon polymers containing unsaturated double bonds for the aforementioned purpose. In the catalysts available for these methods of polymer hydrogenation, (1) carried heterogeneous catalysts having such metals as nickel, platinum, palladium, and ruthenium deposited on carriers such as carbon, silica, alumina, silica-alumina, and diatomaceous earth and (2) so-called Ziegler type homogeneous catalysts obtained by causing organic acid salts of nickel, cobalt, iron, or chromium or acetylacetone salts to react with a reducing agent such as an organic aluminum compound in a solvent are popular.
The former carried heterogeneous catalysts of (1) generally exhibit lower levels of activity than the Ziegler type homogeneous catalysts. To provide effective hydrogenation, therefore, they require hydrogenation to proceed under harsh conditions such as elevated temperatures and high pressure. On a polymer which is to be hydrogenated, no hydrogenation proceeds unless the polymer is exposed to such a catalyst as described above. In the hydrogenation of a polymer, unlike that of a low molecular weight compound, the contact of the polymer with the catalyst is difficult because of the influences of high viscosity acquired by the reaction system and steric hindrance caused by the polymer chain. For the polymer to be efficiently hydrogenated, therefore, the catalyst is required to be used in too large an amount so as to render the hydrogenation uneconomical and the reaction of hydrogenation is also required to be carried out at elevated temperatures under high pressure, namely under conditions liable to induce decomposition and gelation of the polymer. In the hydrogenation of a copolymer of a conjugated diene and a vinyl-substituted hydrocarbon, selective hydrogenation of the unsaturated double bonds in the conjugated diene units of the copolymer generally becomes difficult because the hydrogenation also occurs on the aromatic nucleus portion of the copolymer.
In contrast, the latter Ziegler type homogeneous catalysts of (2) are characterized by exhibiting generally higher levels of activity, effecting hydrogenation at lower rates of use, and providing hydrogenation at lower temperatures under lower pressure than the carried heterogeneous catalysts because the reaction of hydrogenation generally proceeds in a homogeneous system. Depending on the selection of the conditions of hydrogenation, they are also capable of effecting preferrential hydrogenation of the unsaturated double bonds of conjugated diene units in copolymers of conjugated dienes and vinyl-substituted aromatic hydrocarbons to a fairly large extent. The Ziegler type homogeneous catalysts, however, generally suffer from a disadvantage that these catalysts do not easily manifest the hydrogenating activity unless they are prepared immediately before use by mixing their components and reducing the resultant mixtures, they are deficient in repeatability of behavior, and the exhibit inferior stability despite the reduction given preparatorily thereto, making it indispensable for these catalysts to be prepared immediately before actual use each time they are employed in the hydrogenation. Further, particularly in the hydrogenation of a copolymer of a conjugated diene and a vinyl-substituted aromatic hydrocarbon, thorough selectivity of the hydrogenation of the unsaturated double bonds of conjugated diene units relative to the aromatic nucleus portion has not yet been fulfilled. Under the conditions which permit thorough hydrogenation of the unsaturated double bonds of conjugated diene units, the aromatic nucleus portion of the copolymer is inevitably hydrogenated to some extent. Conversely, under the conditions which completely prevent hydrogenation of the aromatic nucleus portion of the copolymer, the hydrogenation of the unsaturated double bonds of conjugated diene units does not occur at an amply high ratio. In this circumstance, development of a catalyst capable of selectively hydrogenating the unsaturated double bonds of conjugated diene units has been highly desired.
Further, the existing Ziegler type catalysts for hydrogenation are expensive and they have a disadvantage that the expulsion of residual catalysts from the hydrogenation products calls for a complicated removal process. For the purpose of effecting the hydrogenation advantageously from the economic point of view, it is earnestly desired to develop a highly effective hydrogenation catalyst capable of manifesting the desired effect at a rate of consumption low enough to obviate the necessity for deliming or a catalyst capable of permitting very easy removal thereof.